An optical communication technique has been required to strictly control a dispersion characteristics and long-term reliability for meeting the demand of increased communication capacity primarily needed by a wavelength division multiplexing in recent years. For this reason, an optical fiber cable has also been required to control a polarization-mode dispersion characteristics, or the increase of transmission loss on the occasion of use in a high temperature high humidity environment.
An optical fiber undergoes an increase in the transmission loss due to the microbending generated by various external stresses. Therefore, in order to protect an optical fiber from external stresses, the optical fiber is generally provided with a coating having a double layer structure composed of a soft layer and a hard layer. The inner layer that is brought into contact with a glass optical fiber is made into a buffer layer (hereinafter, referred to as a primary coating layer) by using a soft resin having a relatively low Young's modulus, and the outer layer is made into a protective layer (hereinafter, referred to as a secondary coating layer) by using a hard resin having a relatively high Young's modulus. In general, a resin having a Young's modulus of 0.3 Pa to 3 MPa is used for the primary coating layer, and a resin having a Young's modulus of 500 MPa to 2000 MPa is used for the secondary coating layer. For the primary coating layer and the secondary coating layer, for example, UV curable resins containing a urethane acrylate-based oligomer or an epoxy acrylate-based oligomer as a main component are used.
In a method of manufacturing an optical fiber, an optical fiber preform containing quartz glass as a main component is melted by heating in a fiber drawing furnace, and a glass optical fiber is drawn from the preform. Then, a liquid UV curable resin is applied to the drawn glass optical fiber using a coating die. Subsequently, the resultant optical fiber is irradiated with UV rays to cure the UV curable resin. As such, in the manufacturing step for an optical fiber, the circumference of the drawn glass optical fiber is immediately coated with a coating resin in order to prevent a decrease in the strength of the optical fiber. By such a method, the glass optical fiber is coated with the primary coating layer and the secondary coating layer, and thereby an optical fiber is manufactured.
Furthermore, in the subsequent step, the circumference of the optical fiber thus obtained is coated with a colored layer made of a colored resin, and thereby a colored optical fiber is manufactured. There are no particular limitations on the coloring of the colored layer, but for example, an UV curable resin added with a coloring agent is used.
In this description, such a glass optical fiber coated with a primary coating layer and a secondary coating layer is referred to as a coated optical fiber; a coated optical fiber whose circumference is further coated with a coating layer made of a colored resin is referred to as a colored optical fiber; furthermore, multiple colored optical fibers arranged parallel to each other in a plane and collectively covered with a ribbon resin are referred to as an optical fiber ribbon.
Regarding a method of suppressing the increase in the transmission loss for the case of using a coated optical fiber in a high temperature high humidity environment, Patent Document 1 discloses a method of setting the relaxation modulus of the secondary coating layer to 400 MPa or less.
An optical fiber ideally is to be completely round in cross section. However, in fact the cross section of the optical fiber includes all sorts of asymmetry such as the cladding surface shape of the optical fiber cross-section deviating from a complete circle and concentricity error. The asymmetry of the optical fiber induced by production facilities and production conditions, so that the asymmetry tends to caused to continue in the longitudinal direction as well as at a cross section of the optical fiber. The propagation of light through the optical fiber with non-circularity asymmetry produces a difference in propagation velocity between that of X and Y polarization mode to cause dispersion. This is known as a polarization-mode dispersion (PMD).
In regard to the polarization-mode dispersion of an optical fiber, in order to suppress this, there is known a method of preventing the asymmetry present in the optical fiber cross-section to continue in the longitudinal direction, by imparting predetermined spin to the optical fiber when drawing fiber from an optical fiber preform. Thereby, an optical fiber in which the velocity of the X polarization mode is approximately equalized with the propagation velocity of the Y polarization mode, and thus the polarization-mode dispersion is reduced, and a method for producing such optical fiber have been proposed. On the other hand, since the cross-section of an optical fiber ribbon is asymmetric in the thickness direction and the width direction, there is a problem that the stress applied to each optical fiber in the thickness direction is different from the stress in the width direction. Each optical fiber tend to have the polarization-mode dispersion increased due to the asymmetry in stress, and in optical fiber ribbons and optical fiber cables obtained by gathering optical fiber ribbons, the polarization-mode dispersion may be increased.